Chemical processing plants for the drying of a liquid feed by means of its spraying and evaporation have been known and successfully practiced in special applications, particularly for the preparation of aluminous porcelains, since the last quarter of the nineteenth century.
Since the beginning of the twentieth century extensive application of the spray drying process in a wide variety of industries has been evident, e.g., in drying milk, soaps, detergents, pharmaceuticals, organic and inorganic chemicals, etc.
There are many known engineering monographs describing this process and its peculiarities with respect to the spraying equipment design. An example is W. R. Marshall's Atomization and Spray Drying, published by the American Institute of Chemical Engineers, 1954, or Ceramic Monographs--Handbook of Ceramics, 1980 Verlag Schmid GmbH.
The basic principles of the spray drying process include preparation of the liquid feed, which is then atomized into a spray. This spray, presented as a cloud of fine droplets, is projected into a stream of a hot gas which is contained within a cylindrical chamber with a conical or flat base. Drying by evaporation from the large exposed surface of the spray is rapid, and the vapor driven off is extracted from the chamber by means of cyclones, wet scrubbers or other appropriate equipment. The drying process is terminated when moisture content in the dried particles is reduced to the desired value and the particulated product is discharged from the chamber.
One of the additional advantages of spray drying systems is associated with their versatility in that they operate both in a continuous cycle without interruption as long as wet feed is supplied, as well as in a batchwise manner if so desired.
The main disadvantage of spray drying equipment implementing the transfer of heat energy to liquid feed by means of direct contact with hot gas is associated with the fact that it is usually suitable only for those materials which are not heat sensitive or readily oxidized, like minerals, inorganic oxides, bentonite, calcite, etc. For some materials, like some foodstuffs, pharmaceuticals, penicillin, blood plasma, and many others, this way of heat transfer is unsuitable, and spray dryers which employ hot gases or other means that come in direct contact with the dried feed, cannot be utilized.
It is common to dry heat sensitive or oxidizable materials by means of indirect batch dryers in which heat is transferred to the wet material via a retaining wall, and there is no contact between the vaporized liquid medium and heating medium. An example of these dryers is the so-called agitated pan dryer which operates atmospherically or under a vacuum, and usually handles only small amounts of nearly any wet solid, that is, liquids, slurries, pastes, etc.
Another common type of indirect dryer suitable for processing heat sensitive materials is the freeze dryer in which wet material is frozen prior to drying. Since it is necessary to maintain a very high vacuum when drying in the frozen state, freeze dryers are rather expensive installations due to the complex and sophisticated vacuum systems which they employed. Use of this type of dryer is, in most cases, limited to pharmaceuticals, fine chemicals and other related products which cannot be dried by any other means.
Another type of indirect dryer which is applicable for heat sensitive materials is the vacuum rotary dryer or vacuum shelf dryer. In vacuum rotary dryers the wet material is agitated in a horizontal stationary shell, the vacuum not always being necessary. In vacuum shelf dryers there is no agitation; the wet material is heated by contact with steam-heated or hot water-heated shelves on which the material lies.
The main disadvantage of indirect dryers is their basically reduced efficiency in terms of output which is associated with the batch mode of operation. Because of the long holdup required for internal diffusion of heat or moisture, a long heating cycle is necessary for achieving the desired moisture content.
There is also known method of producing a thermoreactive resin moulding material which is described in the Japanese laid-open patent application (Kokai) No. 58-13634 assigned to Matsushita Denki Co., Ltd. This method includes the step of spraying water-soluted urea resin syrup under low pressure with the simultaneous dehydration by heating of the condensation polymer. The drying system implementing this method includes a dehydrator in which the preheated syrup is sprayed by nozzle, and evaporates by virtue of the heat, supplied to the interior of the dehydrator via its walls. The moisture and solutes are evacuated from the dehydrator by a vacuum pump, leaving the dried urea resin which is collected in the lower part of the dehydrator in the form of particles.
Moisture and solutes are evacuated via an outlet port supplied in the side wall of the dehydrator in the vicinity of the spraying nozzle. The disposition of the outlet opening for evacuated moisture close to the spraying nozzle might be associated with certain limitations upon the relationship between the velocity submitted by the spraying nozzle to the atomized feed and the velocity submitted by the vacuum pump to driven-off solutes.
If the velocity of particles moving together with the sprayed feed via the heating zone towards the lower part of the dehydrator is too slow, or the velocity of the driven-off solute is too fast, the major part of the feed might be driven off by the pump from the dehydrator before the drying process has been completed. This situation might arise, e.g., when the feed is atomized into very fine particles by the ultrasonic nozzle, since the ultrasonically atomized spray moves with a velocity of several tenths cm per second, while the solute, driven off by the vacuum pump, moves with velocities faster by an order of magnitude.
This assumption is supported by the indication which can be found in this application, that the final product collected in the bottom of the dehydrator consists of relatively coarse, 60 mesh particle size, particles. It should therefore be concluded that the drying system disclosed in the Japanese application inevitably becomes limited to only a certain type of atomizing means and therefore to a particular size of final product, ensuring the optimal ratio between velocities of sprayed feed and driven-off solute.